It wasn't stupid at all!
I just had a argument with another guy with a hippo in a nano tank. That tang grows from 1" to 3-4" in about 6 months, then rapidly gets bigger from there. If he starts out tiny he can probably keep him in there 6 months to a year. Sailfins get just as big.

My hippo is in my 300 gallon reef and he is getting too big for that tank IMO. The sailfin is already 5"

WHOA!!! I just looked and relized it says 180 gal for sailfin ! For some reason i thought it said 90. Whydid you even answer such a stupid question?!

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Yeah, I don;t think people realize just how big those cute little sailfin tangs grow to. I once got one in from my supplier that was too tall for a standard 5' long 100 gallon tank. When it opened it's fins full it stuck out of the tank. It was so big that we didn't have a net big enough to take the fish out of the acclimation set up. We had to use a large garbage bag to move him. It wound up quickly being donated to the Miami Seaquarium because back then, nobody was keeping big tanks in their homes. ( It was originally ordered for a 300 gal tank for the store but that tank was redecorated because it took months for the fish to finally get caught and shipped to me so it no longer could go in there. )
Needless to say, a 180 is probably marginal for a fully grown Sailfin.

If you want tangs, all it takes is a bit of swimming room. They also like rock work that provides hidey holes and safe places to sleep or retreat. Sometimes they can't stand the sight of each other, so it's best there are places out of sight. Constant swimmers, the longer the tank the better. For example, a generic tang will do better in a 200 gallon long tank, than a even bigger high tank that was shorter. They are very easy to raise, even some of the difficult ones. You just need to follow some basic rules and know your tangs. Especially the environment they evolved in. Deep water vs surge zone tangs can make a big difference in how you need to care for them.

If you want tangs, all it takes is a bit of swimming room. They also like rock work that provides hidey holes and safe places to sleep or retreat. Sometimes they can't stand the sight of each other, so it's best there are places out of sight. Constant swimmers, the longer the tank the better. For example, a generic tang will do better in a 200 gallon long tank, than a even bigger high tank that was shorter. They are very easy to raise, even some of the difficult ones. You just need to follow some basic rules and know your tangs. Especially the environment they evolved in. Deep water vs surge zone tangs can make a big difference in how you need to care for them.

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+1 for sure.
So many fish we keep in tanks, in nature, grow way larger than the tanks we try to keep them in. You need to be selective or have a big room for a really big tank.